This invention relates to multilayer circuits comprising liquid crystalline polymers (LCPs) and methods for the manufacture thereof.
As used herein, a circuit material is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, bond plies, conductive layers, resin coated conductive layers, and cover films. These circuit materials each comprise dielectric materials formed from a thermosetting or thermoplastic polymer. The dielectric material in a bond ply, resin covered conductive layer, or cover film may comprise a substantially non-flowable dielectric material, i.e., one that softens or flows during manufacture but not during use of the circuit. The dielectric material in a circuit laminate (i.e., a dielectric substrate), in contrast, is designed to not soften or flow during manufacture or use of the circuit or multi-layer circuit. Dielectric substrate materials are further typically divided into two classes, flexible and rigid. Flexible dielectric substrate materials generally tend to be thinner and more bendable than the so-called rigid dielectric materials, which typically comprise a fibrous web or other forms of reinforcement, such as short or long fibers or fillers.
Circuit laminates further have a conductive layer fixedly attached to a dielectric substrate layer. When a second conductive layer is disposed on the other side of the dielectric layer, the material is often referred to as a double clad circuit laminate. Patterning a conductive layer of a circuit laminate, for example by etching, provides a circuit layer. One or both of the conductive layers of a double clad laminate may be processed to provide circuit layers.
The aforementioned circuit materials and circuits may be combined in various configurations to provide multilayer circuits. “Multilayer circuits” as used herein refers to materials having at least twodielectric layers and at least three conductive layers, wherein at least one of the conductive layers is circuitized, and is inclusive of both subassemblies used to form finished circuits as well as the finished circuits themselves.
In one simple form, a multilayer circuit includes a double clad circuit and a resin coated conductive layer, wherein the dielectric material of the resin coated conductive layer is disposed on a circuit layer of the double clad circuit. In another embodiment, a second resin coated conductive layer is present, wherein the dielectric material of the second resin coated conductive layer is disposed on the second conductive layer (or circuit layer) of the double clad circuit. In still another simple form, a multilayer circuit includes a first circuit and a second circuit joined by a bond ply disposed between the circuit layer of the first circuit and the dielectric substrate of the second circuit. Typically, such multilayer circuits are formed by laminating the circuit(s) and/or circuit material(s) in proper alignment using heat and/or pressure. Bond plies can be used to provide adhesion between circuits and/or between a circuit and a conductive layer, or between two conductive layers. In place of a conductive layer bonded to a circuit with a bond ply, the multilayer circuit may include a resin coated conductive layer bonded directly to the outer layer of a circuit. In such multilayer structures, after lamination, known hole forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
Liquid crystalline polymers are known for use as dielectric layers, bond plies, and in cap layers in circuits and multilayer circuits. Canadian Patent Application No. 2,273,542 to Forcier, for example, describes a circuit laminate made by bonding an adhesive resin-coated copper foil to a thin liquid crystal polymer film, but does not disclose how to make these circuit laminates into multilayer circuits.
It is further known to anneal liquid crystalline polymers prior to use in circuits, as annealing can increase their resistance to high temperatures, as well as properties such as solder resistance and in-plane coefficient of thermal expansion. The term “annealing” as used herein is the process of raising the crystalline to nematic melting point (as defined by the peak endotherm above the glass transition temperature in a differential scanning calorimeter (DSC) measurement) of a liquid crystalline polymer by holding the polymer at a temperature higher than its glass transition temperature, but lower than its melting point, for an extended period of time. As described in a brochure published by Ticona, the manufacturer of VECTRA® liquid crystalline polymer, “the high heat deflection resistance of Vectra® liquid crystalline polymers can further be raised by 30 to 50° C. by thermal after-treatment of the molded parts.” To anneal an liquid crystalline polymer composition with a melting point of 280° C., the brochure suggests heating the oven and parts from room temperature to 220° C. over 2 hours; gradually increasing the temperature from 220° C. to 240° C. over 1 hour; maintaining the temperature at 240° C. for 2 hours; gradually increasing the temperature from 240° C. to 250° C. over one hour; maintaining the temperature at 250° C. for 2 hours; and cooling to room temperature. Similar annealing cycles with different temperature set points for different grades of liquid crystalline polymer with different melting points and glass transition temperatures are also provided. U.S. Pat. No. 6,274,242 discloses use of specific heat treatment schedules to more rapidly anneal liquid crystalline polymer films.
While a variety of materials and methods are known for producing multilayer circuits using liquid crystalline polymers, including annealed liquid crystalline polymers, they suffer from a number of drawbacks that limit their utility. For example, since the temperature limit of conventionally electrically heated presses is less than or equal to about 280° C., it would be highly desirable to use bond plies that soften and flow at the comparatively low temperature of less than about 280° C., and preferably less than about 250° C. However, it is also highly desirable for multilayer circuits to resist temperatures of 260° C. or higher. This is particularly important for soldering operations for device attachment to the multi-layer circuit board. When current commercially available liquid crystalline polymers are used as bond plies, the bond plies re-melt upon soldering or other high temperature operations, which can cause blistering in the bond ply or dimensional distortion of the part.
U.S. Pat. No. 5,259,110 to Bross et al. discloses multilayer printed circuit boards having liquid crystalline polymer dielectric substrate layers. Bross et al. further discloses that several such layers may be bonded together using heat and pressure, and that a bond ply adhesive layer such as a another liquid crystalline polymer layer may be used. However, during fabrication of the multilayer boards of Bross et al., the liquid crystalline polymer layer must be heated at or above its melting point to achieve good adhesion, which can result in distortion or flow of the circuit layers.
U.S. Pat. No. 5,719,354 to Jester et al. discloses the fabrication of multilayer circuits using an liquid crystalline polymer bond ply with a melting temperature of at least 10° C. lower than that of the liquid crystalline polymer circuit layer dielectric material. The use of the lower melting bond ply eliminates or substantially reduces distortion of the circuit layers during lamination, but it also limits the maximum processing temperature of the multilayer circuit during subsequent lamination and/or component soldering to less than or equal to the melting temperature of the bond ply, in order to avoid distortion or blistering and loss of adhesion.
U.S. Pat. No. 6,538,211 to St. Lawrence et al. discloses a multilayer circuit comprising a circuit and a cap layer including a liquid crystalline polymer layer disposed on the circuit. This circuit can be used in adding fine line thin outer layers to a high wiring density multi-layer circuit board. However, if several cap layer are to be added sequentially to the multilayer circuit, cap layers having liquid crystalline polymers with sequentially lower melting points must be used in order to avoid distortion of the previously laminated layers. This can be particularly problematic, as any subsequent processing of the completed multilayer circuit such as soldering must be done at a temperature lower than melting point of the lowest melting point composition of the resin coated foil liquid crystalline polymer resin layers.
Accordingly, there remains a need in the art for a method for the manufacture of a multilayer circuit having a thermoplastic liquid crystalline polymer layer, wherein the method allows both assembly of the layers at temperatures usable with conventional electrically heated presses, and subsequent processing at higher temperatures without warping, blistering, and/or loss of bond.